Previous research found a correlation between the body muscle-to-fat ratio (BMFR) and whole-body insulin sensitivity. In this study, researchers analyzed BMFR cut-off values for impaired insulin sensitivity in untreated type 2 diabetes (T2D) patients based on sex. A total of 101 patients with T2D were included: 66 males and 35 females. Whole-body insulin resistance was determined with a hyperinsulinemic–euglycemic clamp examination to measure the steady-state glucose infusion rate (M value). The M/I value was calculated by dividing the M value by the steady-state serum insulin value, and the presence of insulin resistance was determined using an M/I ratio <9.0. Receiver operating characteristics (ROC) analysis was implemented to establish the optimal BMFR cut-off value. In males, the cut-off value of the BMFR for insulin resistance was 2.75 (area under the curve [AUC]=0.83, sensitivity 75%, and specificity 76%, P<0.001), and in females, 1.65 (AUC=0.87, sensitivity 84%, and specificity 81%, P<0.001). In both males and females, simple linear regression analysis observed a significant correlation between BMFR and the M/I value (males, B=0.77, P<0.01; females, B=0.83, P<0.01).